74 
PALEONTOLOGY OF NEW YORK. 
Diagnosis. Shells having the same general external characters as Mebista. 
Valves convex, often inflated, cardinal areas obscure. The umbo of the pedicle- 
valve is incurved at maturity, concealing most, if not all of the foramen; in 
early stages of growth, however, the beak is more erect and exposes the delti- 
dial plates in an elementary condition of development. The anterior margin 
of the shell is sinuate, and usually there is a sinus on the pedicle-valve, with a 
less conspicuous fold on the brachial valve; sometimes both valves bear a low 
sinus, or the sinus on the pedicle-valve may be absent, while the fold on the 
brachial valve is present, thus giving the shell a nasute anterior extension ; 
again, fold and sinus may be absent on both valves. 
In the interior of the pedicle-valve the delthyrium is wide, its margins being 
thickened into dental ridges. The teeth are conspicuous, often much thickened 
and curved backward at their tips, interlocking with the opposite valve in such a 
manner as to make a very Arm articulation. The teeth are supported by lamellm 
which rest upon the bottom of the valve, and are continued for a short distance 
about the posterior margin of the muscular impression. In old shells this por¬ 
tion of the valve becomes greatly thickened, the muscular impression corres¬ 
pondingly deepened, and the identity of the dental lamellae is obscured by 
their becoming merged with the substance of the valve. The pedicle-cavity 
is deep and frequently shows a strong muscular scar. The impression of the 
diductor muscles is subquadrate-ovate or subtriangular in outline, very strongly 
impressed and usually clearly divisible into its two lateral components. The 
central adductor scar is faint, but linear when retained. The lateral scars are 
deeply striated longitudinally. The anterior margin of the muscular area is 
frequently obscure but is not infrequently a ridge from which radiate flne, 
anastomosing pallial sinuses. In the post-lateral regions the ovarian sinuses 
are sometimes retained. 
In the brachial valve the beak is depressed and sometimes obscured by the 
incurvature of the umbo of the opposite valve. The dental sockets are nar¬ 
row and divergent. The hinge-plate is subject to some unessential variation 
in form. Usually it is triangular, concave on the upper surface, and divided 
into two lobes by a median groove. The crura take their origin from just 
